1. Field of the Invention
The present invention relates to the design and operation of an ion source for use in the ion implantation of semiconductors, and for the modification of the surfaces of materials. The ion source can be retrofitted into the existing fleet of ion implanters currently used in the manufacture of semiconductor devices, particularly those used in Complementary Metal-Oxide Semiconductor (CMOS) manufacturing. The ion source is specifically designed to accommodate the use of new solid feed materials such as decaborane (B10H14) and Trimethyl Indium (TMI), which vaporize at sufficiently low temperatures that currently available ion implant ion sources cannot use them. Indeed, the currently available (ion sources result in disassociation of decaborane when that material is introduced into tLem. The ion source has an integral low-temperature vaporizer, and a means of introducing the vaporized feed material into an ionization chamber which is also temperature controlled by the vaporizer. The feed material is ionized by a variable energy, variable current, wide-area electron beam which passes through the ionization chamber, but is largely prevented from interacting with the chamber walls. The ion source also incorporates a gas feed for introducing gaseous materials from pressurized gas cylinders.
2. Description of the Prior Art
Ion implantation is a key enabling technology in the manufacture of integrated circuits (IC""s). In the manufacture of logic and memory IC""s, ions are implanted into silicon or GaAs wafers to form the transistor junctions, and to dope the well regions of the pn junctions. By varying the energy of the ions, their implantation depth into the silicon can be controlled, allowing three-dimensional control of the dopant concentrations introduced by ion implantation. The dopant concentrations control the electrical properties of the transistors, and hence the performance of the IC""s. A number of different electrically active dopant materials are used, including As, B, P, In, Sb, Be, and Ga. Many of these materials can be obtained in gaseous chemical form, for example as AsH3, PH3, BF3, PH3, and SbF5. The ion implanter is a manufacturing tool which ionizes the dopant-containing feed materials, extracts the dopant ions of interest, accelerates the dopant ion to the desired energy, filters away undesired ionic species, and then transports the dopant ion of interest to the wafer. Thus, the following variables must be controlled in order to achieve the desired implantation profile for a given implantation process:
Dopant feed material (e.g., BF3 gas)
Dopant ion (e.g., B+)
Ion energy (e.g., 5 keV)
Chemical purity of the ion beam (e.g.,  less than 1% contaminants)
Energy purity of the ion beam (e.g.,  less than 2% FWHM).
An area of great importance in the technology of ion implantation is the ion source. FIG. 1 shows the xe2x80x9cstandardxe2x80x9d technology for commercial ion sources, namely the xe2x80x9cEnhanced Bernasxe2x80x9d ion source. This type of source is commonly used in high current, high energy, and medium current ion implanters. The ion source a is mounted to the vacuum system of the ion implanter through a mounting flange b which also accommodates vacuum feedthroughs for cooling water, thermocouples, dopant gas feed, N2 cooling gas, and power. The dopant gas feed c feeds gas into the arc chamber d in which the gas is ionized. Also provided are dual vaporizer ovens e, fin which solid feed materials such as As, Sb2O3, and P may be vaporized. The ovens, gas feed, and cooling lines are contained within a cooled machined aluminum block g. The water cooling is required to limit the temperature excursion of the aluminum block g while the vaporizers, which operate between 100 C. and 800 C., are active, and also to counteract radiative heating by the arc chamber d when the source is active. The arc chamber d is mounted to, but in poor thermal contact with, the aluminum block g. The ion source a is an arc discharge source, which means that it operates by sustaining a continuous arc discharge between an immersed hot-filament cathode h and the internal walls of the arc chamber d. Since this arc can typically dissipate in excess of 300 W, and since the arc chamber d cools only through radiation, the arc chamber can reach a temperature in excess of 800 C. during operation.
The gas introduced to arc chamber d is ionized through electron impact with the electron current, or arc, discharged between the cathode h and the arc chamber d. To increase ionization efficiency, a uniform magnetic field i is established along the axis joining the cathode h and an anticathode j by external Helmholz coils, to provide confinement of the arc electrons. An anticathode j (located within the arc chamber d but at the end opposite the cathode h) is typically held at the same electric potential as the cathode h, and serves to reflect the arc electrons confirmed by the magnetic field i back toward the cathode h and back again repeatedly. The trajectory of the thus-confined electrons is helical, resulting in a cylindrical plasma column between the cathode h and anticathode j. The plasma density within the plasma column is typically high, on the order of 1012 per cubic centimeter; this enables further ionizations of the neutral and ionized components within the plasma column by charge-exchange interactions, and also allows for the production of a high current density of extracted ions. The ion source a is held at a potential above ground (i.e., the silicon wafer potential) equal to the accelerating voltage Va of the ion implanter: the energy of the ions E as they impact the wafer substrate is given by E=qVa, where q is the electric charge per ion.
The cathode h is typically a hot filament or indirectly-heated cathode, which thermionically emits electrons when heated by an external power supply. It and the anticathode are typically held at a voltage Vc between 60V and 150V below the potential of the ion source Va. High discharge currents D can be obtained by this approach, up to 7A. Once an arc discharge plasma is initiated, the plasma develops a sheath adjacent to the surface of the cathode h (since the cathode h is immersed within the arc chamber and is thus in contact with the resulting plasma). This sheath provides a high electric field to efficiently extract the thermionic electron current for the arc; high discharge currents can be obtained by this method.
The discharge power P dissipated in the arc chamber is P=DVc, or hundreds of watts. In addition to the heat dissipated by the arc, the hot cathode h also radiates power to the arc chamber d walls. Thus, the arc chamber d provides a high temperature environment for the dopant plasma, which also boosts ionization efficiency relative to a cold environment by increasing the gas pressure within the arc chamber d, and by preventing substantial condensation of dopant material on the hot chamber walls.
If the solid source vaporizer ovens e or f are used, the vaporized material feeds into the arc chamber d through vaporizer feeds k and 1, and into plenums m and n. The plenums serve to diffuse the vaporized material into the arc chamber d, and are at about the same temperature as the arc chamber d. Radiative thermal loading of the vaporizers by the arc chamber also typically prevents the vaporizers from providing a stable temperature environment for the solid feed materials contained therein below about 100 C. Thus, only solid dopant feed materials that both vaporize at temperatures  greater than 100 C. and decompose at temperatures  greater than 800 C. can be vaporized and introduced by this method.
A very significant problem which currently exists in the ion implantation of semiconductors is the limitation of ion implantation technology to effectively implant dopant species at low (e.g., sub-keV) energies. One critically important application which utilizes lowenergy dopant beams is the formation of shallow transistor junctions in CMOS manufacturing. As transistors shrink in size to accommodate the incorporation of more transistors per IC, the transistors must be formed closer to the silicon surface. This requires reducing the velocity, and hence the energy, of the implanted ions. The most critical need in this regard is the implantation of low-energy boron, a p-type dopant. Since boron atoms have low mass, at a given energy they penetrate deeper into the silicon than other p-type dopants, and must therefore be implanted at lower energies. Ion implanters are inefficient at transporting low-energy ion beams due to the space charge within the ion beam causing the beam profile to grow larger (beam blow-up) than the implanter""s transport optics, resulting in beam loss through vignetting. In addition, known ion sources rely on the application of a strong magnetic field in the source region. Since this magnetic field also exists in the beam extraction region of the implanter, it deflects the lowenergy beam and substantially degrades the emittance properties of the beam, further reducing beam transmission through the implanter. For example. at 500 eV transport energy, many ion implanters currently in use cannot transport enough boron beam current to be useful in manufacturing; i.e., the wafer throughput is too low.
Recently, a new enabling technology has been pursued to solve the problem of low-energy boron implantation: molecular beam ion implantation. Instead of implanting an ion current I of atomic B+ ions at an energy E, a decaborane molecular ion, B10Hx+, is implanted at an energy 10xc3x97E and an ion current of 0.10xc3x97I. The resulting implantation depth and dopant concentration (dose) of the two methods have been shown to be equivalent, but the decaborane implantation technique has significant advantages. Since the transport energy of the decaborane ion is ten times that of the dose-equivalent boron ion, and the ion current is one-tenth that of the boron current, the space charge forces responsible for beam blowup and the resulting beam loss are much reduced relative to monatomic boron implantation.
While BF3 gas can be used by conventional ion sources to generate B+ ions, decaborane (B10H14) must be used to generate the decaborane ion B10Hx+. Decaborane is a solid material which has a significant vapor pressure, on the order of 1 Torr at 20 C., melts at 100 C., and decomposes at 350 C. It must therefore be vaporized below 100 C., and operate in an ion source whose local environment (walls of the arc chamber and components contained within the arc chamber) are below 350 C. In addition, since the B10H14 molecule is so large, it can easily disassociate (fragment) into smaller components, such as elemental boron or diborane (B2H6), when subject to charge-exchange interactions within a dense plasma. Therefore, in order to preserve the B10Hx+ ion, the plasma density in the ion source must be low. Also, the vaporizers of current ion sources cannot operate reliably at the low temperatures required for decaborane. This is due to radiative heating from the hot ion source to the vaporizer causing thermal instability, and the fact that the vaporizer feed lines k, l easily become clogged with decomposed vapor as the decaborane vapor interacts with their hot surfaces. Hence, the prior art of implanter ion sources is incompatible with decaborane ion implantation.
The present invention provides an improved means for efficiently:
Vaporizing decaborane;
Delivering a controlled flow of vaporized decaborane into the ion source;
Ionizing the decaborane into a large fraction of B10Hx+;
Preventing thermal dissociation of decaborane;
Limiting charge-exchange induced fragmentation of B10Hx+;
Operating the ion source without the use of an applied magnetic field, which improves the emittance properties of the beam.
Uses a novel approach to produce electron impact ionizations without the use of an arc discharge, by incorporation of an externally generated electron beam which passes through the ionization chamber.
In addition, the present invention is compatible with current ion implantation technology, such that the ion source can be retrofitted into the existing fleet of ion implanters currently used in the manufacture of semiconductor devices.